August 2026
Togo, Fumiharu; Matsukuma, Seiya
Sleep Epidemiology
Sleep timing is linked to affect and mental health; however, evidence has largely been limited to between-person associations. We therefore examined the within- and between-person associations between the nighttime sleep midpoint and next-morning affect during daily life in young adults. Healthy young adults wore a wrist actigraph for 15 consecutive days and underwent smartphone-based assessments of positive affect (PA), negative affect (NA), and state anxiety (Anxiety) immediately before getting into bed and within 15 min of waking. Nightly sleep midpoint and duration were derived using actigraphy. Linear mixed-effects models were used to decompose within-person (person-mean-centered) and between-person (person-means) associations, adjusted for sex and age (Model 1) and additionally adjusted for weekday–weekend status and the corresponding prior-evening outcome (Model 2). Thirty participants provided usable actigraphy and smartphone-based data. Nighttime sleep midpoint showed opposite patterns across levels: within individuals, a later sleep midpoint was associated with lower next-morning NA and Anxiety, whereas between individuals, a later mean sleep midpoint was associated with higher average NA and Anxiety. These timing–affect associations remained essentially unchanged after adjusting for weekday–weekend status and prior-evening affect. In contrast, nighttime sleep duration was not associated with next-morning PA, NA, or Anxiety at either the within- or between-person level. These exploratory findings from this observational study suggest that associations between sleep timing and affect may differ in direction across within- and between-person levels. Replication in larger observational studies and further examination in experimental studies are needed.
Togo, Fumiharu; Matsukuma, Seiya Within- and between-person associations of actigraphy-derived nighttime sleep midpoint with the next-morning affect in young adults: A 15-day intensive longitudinal study Sleep Epidemiology 2026. https://www.sciencedirect.com/science/article/pii/S2667343626000326.